Electricity supply organisations prefer to operate generating stations at constant high output to make the best use of both plant and fuel. But consumer demand normally falls at night, so some short-term storage systems have been built to absorb energy at night and release it when demand is high during the day. For example, a reversible plant comprising motor/dynamo plus water pump/turbine can absorb electrical power by pumping water to a higher reservoir overnight, and generate hydroelectric power from the raised water next day. Very large volumes of water are needed for such systems and, for social and geological reasons, suitable sites are rare. An essential requirement of any practicable storage system is that a high proportion of the energy absorbed should be recoverable.
In this invention of improved means of energy storage, energy is absorbed by refrigerating and liquefying air, and liquid nitrogen and liquid oxygen, the main atmospheric constituents, are stored at temperatures considerably below atmospheric. The cold stored liquid gases are subsequently used, in conjunction with a source of heat at a higher temperature such as the atmosphere, to drive a heat engine and thus make energy available when required.
A cold liquid gas such as nitrogen or oxygen will not yield mechanical energy if quite isolated, but can do so in combination with a source of heat at a higher temperature. One such source is the atmosphere, in which a virtually infinite amount of ambient temperature heat is available at no cost. Two basically different methods may be employed.
(A) The cold liquid may be boiled, for example by blowing air over pipes containing it, thus producing cold gas which can be further warmed and pressurised by atmospheric heat and used to drive an open cycle engine such as a turbine, hence making mechanical energy available. PA1 (B) Instead of passing atmospheric heat directly to the boiling liquid as in (A), the liquid may be cirulated around the "cold end" of a closed cycle "heat engine" with separate working fluid (such as a Stirling engine using trapped helium as working fluid), the engine "hot end" being held close to the atmospheric temperature by blowing air over it. Although in such circumstances no part of the engine will be at a temperature above atmospheric, it will run because of the difference of temperature between its "cold" and "hot" ends. Operation of the engine will deliver heat at a low temperature to boil the cold liquid, producing cold gas in the same quantity as in (A) which can likewise be warmed and used to drive a turbine and so make mechanical energy available. However, there is a big advantage in delivering heat for boiling the liquid via the closed cycle heat engine, because the engine yields additional mechanical energy (by virtue of its operation) in the very process of delivering the heat. The lower the temperature of the cold end of the engine the greater the amount of such energy, so to maximise its mechanical energy output the closed cycle engine should deliver heat at a temperature only marginally above that of the cold liquid (which is sufficient to promote boiling).
Thermodynamics shows that the closed cycle engine of (B) above yields about twice as much mechanical energy as the turbine, so that overall method (B) yields three times as much energy as method (A) from the same amount of liquid nitrogen or oxygen. (The extra mechanical energy comes from transforming more heat energy from the atmosphere). The quantitative distinction between methods (A) and (B) is crucially important to any practical large-scale energy storage system based on the invention, because a high proportion of the energy absorbed must be recovered. Moreover if "waste heat" at above atmospheric temperature is available, it is beneficial to use it (instead of the air-borne atmospheric heat assumed so far) for pressurising gas for the turbine and at the "hot end" of the closed cycle engine, because more mechanical energy will then be yielded per gallon of liquid gas supplied. The higher the temperature of the waste heat the greater will be the benefit of using it, but even very low grade waste heat (say at 15.degree. C. above atmospheric temperature, which is useless for other purposes) can be usefully employed at the "hot end" of the closed cycle engine or in pressurising gas for the turbine. It is important to notice that the engine and turbine basically operate because of the cold gas supplied at substantially below atmospheric temperature, not because of any waste heat: by difinition, "waste" heat at a plant cannot be used on its own.
The invention employs a complete reversal of normal heat engine practice (in which the upper temperature is maintained substantially above atmospheric by burning fuel, and the lower kept close to atmospheric by cooling with air or water). Heat engines operating in this new reversed mode, with their "hot" and "cold" ends respectively at temperatures generally close to, and considerably below, atmospheric will often be termed "cold engines" in this Specification; and the cold gas required for operation of a cold engine will often be termed a "cold energy" source.
It is an object of this invention to provide a method for absorption of surplus energy output from an electricity generating station, for temporary storage of energy, and for making available for use when subsequently required a maximum proportion of the energy absorbed. It is a further object of the invention to separate oxygen from air and use it to improve combustion efficiency at an electricity generating station, as described later. The invention may be modified, by those skilled in the art, to provide a method for absorption and temporary storage of energy from sources other than normal electricity generating stations (for example, energy from windmills or arrays of solar power collectors) and for making available for subsequent use a maximum proportion of the energy absorbed. The invention may also be modified to release energy in a different form from that absorbed: for example, although an atomic power plant is too large to be fitted to a vehicle, it can be used to liquefy atmospheric nitrogen which can be stored and subsequently used as a cold energy source to drive a vehicle by means of a closed cycle cold engine or an open cycle turbine or piston engine.